The U.S. Geological Survey, in cooperation with the Eastern Municipal Water District, the Metropolitan Water District of Southern California, and the Orange County Water District, has completed a detailed study of the Hemet groundwater basin. The quantity of ground water stored in the basin in August 1992 is estimated to be 327,000 acre-feet. Dissolved-solids concentration ranged from 380 to 700 mg/L (milligrams per liter), except in small areas where the concentration exceeded 1,000 mg/L. Nitrate concentrations exceeded the U.S. Environmental Protection Agency Maximum Contaminant Level (MCL) of 10 mg/L nitrate (as nitrogen) in the southeastern part of the basin, in the Domenigoni Valley area, and beneath a dairy in the Diamond Valley area.

Seven sites representing selected land uses-- residential, turf grass irrigated with reclaimed water, citrus grove, irrigated farm, poultry farm, and dairy (two sites)--were selected for detailed study of nitrogen geochemistry in the unsaturated zone. For all land uses, nitrate was the dominant nitrogen species in the unsaturated zone.

Although nitrate was seasonally present in the shallow unsaturated zone beneath the residential site, it was absent at moderate depths, suggesting negligible migration of nitrate from the surface at this time. Microbial denitrification probably is occurring in the shallow unsaturated zone. High nitrate concentrations in the deep unsaturated zone (greater than 100 ft) suggest either significantly higher nitrate loading at some time in the past, or lateral movement of nitrate at depth.

Nitrate also is seasonally present in the shallow unsaturated zone beneath the reclaimed-water site, and (in contrast with the residential site), nitrate is perennially present in the deeper unsaturated zone. Microbial denitrification in the unsaturated zone and in the capillary fringe above the water table decreases the concentrations of nitrate in pore water to below the MCL before reaching the water table.

Pore water in the unsaturated zone beneath the citrus grove site contains very high concentrations of nitrate. Even though there are zones of microbial denitrification, nitrate seems to be migrating downward to the water table.

The presence of a shallow perched-water zone beneath the irrigated-farm site prevents the vertical movement of nitrate from the surface to the regional water table. Above the perched zone, nitrate concentrations in the unsaturated zone are variable, ranging from below the MCL to four times the MCL. Periodically, nitrate is flushed from the shallow unsaturated zone to the perched-water zone.

The unsaturated zone pore-moisture quality could not be adequately addressed because of the very dry conditions in the unsaturated zone beneath the poultry-farm site. Surficial clay deposits prevent water from percolating downward.

At the two dairy sites, nitrate loading in pore water at the surface was very high, as great as 7,000 mg/L. Microbial denitrification in the unsaturated zone causes such concentrations to decrease rapidly with depth. At a depth of 20 ft, nitrate concentration was less than 100 mg/L. In areas where the depth to water is less than 20 ft, nitrate loading to ground water can be very high, whereas in areas where depth to water is greater than 100 ft, most of the nitrate is microbially removed before reaching the water table.